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Apparently a private school is experiencing an outbreak of whooping cough. However, the article said that only 18 of the students in the school (out of 1500 or so) were unvaccinated, yet already 30 have the disease. Presumably some of the infected students were vaccinated. How can this be?

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Per the CDC:

A: Pertussis vaccines are effective, but not perfect. They typically offer good levels of protection within the first 2 years after getting the vaccine, but then protection decreases over time. Public health experts call this ‘waning immunity.’ Similarly, natural infection may also only protect you for a few years.

In general, DTaP vaccines are 80% to 90% effective. Among kids who get all 5 doses of DTaP on schedule, effectiveness is very high within the year following the 5th dose – at least 9 out of 10 kids are fully protected. There is a modest decrease in effectiveness in each following year. About 7 out of 10 kids are fully protected 5 years after getting their last dose of DTaP and the other 3 out of 10 kids are partially protected – protecting against serious disease.

CDC’s current estimate is that in the first year after getting vaccinated with Tdap, it protects about 7 out of 10 people who receive it. There is a decrease in effectiveness in each following year. About 3 or 4 out of 10 people are fully protected 4 years after getting Tdap.

Keeping up-to-date with recommended pertussis vaccines is the best way to protect you and your loved ones.

Given the size of the school and the effectiveness of the vaccine, it appears that ~30 infected kids is within what's expected.

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    Yes, outbreaks of this sort typically start with an unvaccinated index case and are able to spread because of the population of susceptible individuals, but end up infecting some vaccinated children. See my answer on Biology.SE, which discusses other outbreaks since 2000. – De Novo Feb 27 '19 at 21:57
  • @DeNovo: That said, pertussis/whooping cough seems to be an unusually bad case. The vaccine was adjusted for reduced side-effects (from whole cell to acellular) a couple decades ago, and the new version has lower efficacy, and it wanes rapidly. – ShadowRanger Feb 28 '19 at 0:25

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